Which option must be configured to allow SNMP communications and traffic?

An out-of-band bridge domain defines the Layer 2 forwarding for the out-of-band management network but does not, by itself, explicitly allow specific Layer 3 protocols like SNMP through.

In an ACI environment, an out-of-band contract is required to explicitly permit SNMP communications and traffic to and from the out-of-band management EPGs, defining the allowed protocols and ports.

While out-of-band policies exist, the specific mechanism for allowing traffic between EPGs or to external networks is through contracts, not a general 'out-of-band policy.'

An out-of-band management interface provides the physical or logical connection for management traffic, but it is the contract that defines what traffic (e.g., SNMP) is permitted to traverse that interface.